Plasma torch

ABSTRACT

Plasma torch ( 8; 10 ) of improved performances, comprising: an electrode ( 13 ) provided with a respective electrode head ( 18; 37 ); a nozzle ( 14 ); and an outside jacket ( 26 ), there being formed a first cooling circuit ( 20, 21 ) of a coolant for the electrode head ( 18; 37 ) having an end passage ( 22 ), said head being characterised in that it comprises a device ( 25 ) for disposing of the electrode heat, located inside of the first cooling circuit.

The present invention refers to the field of the plasma torches, of thetype employed in plasma furnaces, e.g. utilized for destroying liquidand solid waste products.

With reference to the attached FIGS. 1 and 2, schematically andsectionally depicting two typologies of electric plasma furnace, a firstexample of furnace 1 comprises a container 2 fed with scrap metal, wasteproducts, various slags, toxic and pollutant compounds to be thermallydestroyed, etc., that upon melting form a bath 3 onto the bottom 4 ofthe container 2.

With reference to the sole FIG. 1, at said bottom 4, the container 2comprises a hearth 5 acting as anode, being part of an electric circuitwhose generator is not shown. The container further comprises a top dome6 crossed by a lance 7 employable for injecting liquid and gaseousmaterials, fuel (comburent), and/or destined to destruction. Moreover,said dome is crossed by a plasma torch 8 (single torch) that acts ascircuit cathode, molten and aeriform components being injectedtherethrough. The voltage applied sparks an arc 9 between the proximalend 10 of the torch 8 near to the surface of the bath 3. The highcurrent combined to the high resistance at the arc causes, by Jouleeffect, the production of heat. This entails a very high raise in thetemperature (15.000° C. and above) hence the torch-injected matteracquires the state of a plasma.

With reference to the sole FIG. 2, a second example of twin torchfurnace 1 has the container 2 void of the hearth 5. Instead, a pair oftorches crosses the dome 6. The first torch 8 acts as circuit cathode,whereas the second torch 11 acts as circuit anode. In this case, theplasma electric arc 9 sparks between the distal ends 10, 12 of thetorches 8, 11, and at the surface of the bath 3, when the lance 7 ispositioned between the torches 8, 11.

It is understood that the hereto given description of these furnacetypologies is general, and aimed at explaining the operating conditionsof an anodic or cathodic torch.

Both the abovementioned torches have the same functional and structuraldesign. Each torch substantially consists of an electrode, a nozzle andan outside jacket.

In general, each one of the three components is cooled with deionizedwater. The cooling water is circulated inside the electrode via aninside piping, e.g. of brass, that reverses the water flow.

Examples of this type of torches are taught in U.S. Pat. No. 5,376,767(Heanley et al.), in GB Pat. Appln. 2,355,379 (Tetronics) and in PCTAppln. WO/90/10366 (Tetronics et al.). However, these torches are notfree from drawbacks. In fact, the heads of the nozzles and of theoutside jackets are made of Copper and are soldered to steel pipingsforming the body of these components by electric soldering carried outwith Silver-base alloy. Therefore, during the normal plant operation(with the ignited plasma) the soldering material can melt, causing theloss (spilling) of cooling water inside of the oven and at the plasmazone, with the entailed operation instability and plasma quenching.

Moreover, onto the outside jackets there tends to deposit a layer ofcarbonaceous substance onto which liquid corrosive substances, like e.g.hydrochloric acid, generated during the thermal destruction process canbe adsorbed. Due to the low local temperature of the water-cooled torch,said substances condensate and attack the metal surface of the outsidejacket. Over time, jacket corrosion causes the embrittlement and theconsequent breaking thereof.

Concerning the anodic torch, it suffers from further drawbacks,substantially due to the reduced surfaces onto which the current flowlocalizes, both during the firing phase and during the normal operation,causing microfusions and punctures.

Concerning instead the deionized water-cooling, it causes a remarkableenergy loss, limiting the performance of the entire system.

The technical problem underlying the present invention is to provide aplasma torch overcoming the drawbacks mentioned with reference to theknown art.

This problem is solved by a plasma torch, comprising an electrodeprovided with a respective electrode head, a nozzle and an outsidejacket, there being formed a first cooling circuit of a coolant for saidelectrode head having an end passage, said head being characterised inthat it comprises means for disposing of the electrode heat, locatedinside of the first cooling circuit.

Hereinafter, the present invention will be described according to apreferred embodiment thereof, together with some preferred embodimentsthereof, given by way of a non-limiting example with reference to thefollowing examples and to the attached drawings, in which, besides fromthe abovementioned FIGS. 1 and 2:

FIG. 3 is a longitudinal sectional view of a plasma torch according tothe invention, in particular a cathodic torch;

FIG. 4 is a longitudinal sectional view of another plasma torchaccording to the invention, in particular an anodic torch;

FIG. 5 is a sectional detailed view of the proximal end of the torch ofFIG. 3;

FIG. 6 is a sectional detailed view of the proximal end of the torch ofFIG. 4;

FIG. 7 is a perspective view of a detail of the torches of the precedingFigs.; and

FIG. 8 is a sectional view of the detail of FIG. 7.

With reference to FIGS. 3 and 5, a cathodic torch 8 has a tubular body,having concentric members. Starting, from the central axis of symmetry,inside to outside, the torch comprises an electrode 13 that is insertedin a nozzle 14 made of a tubular pass 16 and of tubular walls 17.

The electrode 13, at the proximal end 10 of the torch 8, comprises anelectrode head 18 ending with a metal coating 19.

Said metallic material coating 19 has a >1600° C. melting temperature,it is suitably made of Tungsten and applied by a plasma spray technique.

Inside of the electrode 13 it is located a first reversing pipe 20, thatextends to the head 18 defining a first toroidal duct 21 between theinside walls of the electrode 13 and the outside wall of the firstreversing pipe 20. At the head 18, the first reversing pipe 20 isspaced, leaving a first end passage 22.

In particular, the first reversing pipe 20 ends in a coolant reversingmember 23 in which it is formed, at the head 18, a toroidal slot 24.Complementarily, the point 18 has, internally to the electrode 13, atoroidal flap 25, formed in the electrode head 18, that is inserted inthe toroidal slot 24, so as to impart an U-shaped course to the endpassage 22.

The first toroidal duct 21 is connected inside of the first reversingpipe 20 by the first end passage 22, thereby defining a first internalcooling circuit that has its ascending section in the first toroidalduct 21 and its descending section inside of the first reversing pipe20.

Hereinafter, for ‘descending’ proximal end-wise is meant, and for‘ascending’ the opposite is meant.

Moreover, the torch 8 comprises an outside jacket 26 defining, with thetubular walls 17, a toroidal gap inside which it is housed a secondreversing pipe 46, located so as to leave, at the proximal end 10 of thetorch 8, a second end passage 27.

Notably, the outside jacket 26 ends in a nozzle head 28 connected to thetubular walls 17 of the nozzle 14. Also the second reversing pipe 46,alike the first ends in a respective second reversing member 29 anddefines said second end passage 27 therat.

The second reversing pipe 46 defines, with the second end passage 27,the tubular walls 17 and the outside jacket 26, a first external coolingcircuit having a toroid-shaped inside descending section 31, and anoutside descending section 33.

The nozzle head 28 comprises, at the proximal end 10 of the torch 8, arefractory material ring 34. Moreover, the nozzle 14 incorporates adispensing member 35 apt to swirl the plasmogen gas that descends alongthe tubular gap 16. The dispensing member 35 is supported onto the bodyof the outside jacket by a ceramics material insulator 36.

The nozzle head of the cathodic torch 8 is tapered.

An anodic torch structured according to the same principles of thepreceding examples will be described hereinafter. Likewise numbers willindicate likewise components.

With reference to FIGS. 4 and 6, an anodic torch 10 has it also atubular body, having concentric members. Starting again from the centralaxis of symmetry, inside to outside, the torch comprises an anodicelectrode 37 that is inserted in a nozzle 14 made of a tubular pass 16and of tubular walls 17.

The anodic electrode 37, at the proximal end 12 of the torch 10,comprises an electrode head 18 having a central port 38, opened on theinside of the anodic electrode 37. Inside of the electrode 37 there islocated a first reversing pipe 20 that extends to the head 18, defininga first toroidal duct 21 between the inside walls of the electrode 13and the outside wall of the first reversing pipe 20. At the head 18, thefirst reversing pipe 20 is spaced, leaving a first end passage 22.

In particular, the first reversing pipe 20 ends in a reversing member 23having, at the head 18, a toroidal slot 24. Complementarily, the point18 has, internally to the electrode 37, a toroidal flap 25 that isinserted in the toroidal slot 24, so as to impart an U-shaped course tothe end passage 22.

From the central port 38, running through the entire electrode body andthereby enabling the flow of plasmogen gas and/or of optional materialsto be thermally destroyed, there concentrically branches out an insidepipe 39 defining, together with the first reversing pipe 20, a secondtoroidal duct 40 connected to the first toroidal duct 21 by the firstend passage 22, thereby defining a first internal cooling circuit thathas its ascending section in the first toroidal duct 21 and itsdescending section in the second toroidal duct 40.

Said first cooling circuit is apt to be crossed by refrigerated fluid,in particular deionized water chilled by a suitable conditioningapparatus.

The head 18 of said anodic electrode 37 is suitably coated with a metalcoating having >0.8 reflectivity, preferably selected from the groupcomprising Molybdenum, Nickel.

Moreover, the anodic torch 10 comprises an outside jacket 26 thatdefines, with the tubular walls 17, a toroidal gap inside which it ishoused a second reversing pipe 46, located so as to leave, at theproximal end 10 of the torch 8, a second end passage 27. Notably, theoutside jacket 26 ends in a nozzle head 28 connected to the tubularwalls 17 of the nozzle 14. Also the second reversing pipe 46, alike thefirst one ends in a respective second reversing member 29 and definessaid second end passage 27 thereat.

The second reversing pipe 46 defines, with the second end passage 27,the tubular walls 17 and the outside jacket 26, a first external coolingcircuit having a toroid-shaped inside descending section 31, and anoutside descending section 33.

The nozzle 14 incorporates a dispensing member 35 apt to swirl theplasmogen gas that descends along the tubular gap 16. The dispensingmember 35 is directly fixed to the tubular walls 17.

The anodic torch 10, at the proximal end thereof, has a diameter uniformto the remaining torch body. Moreover, the nozzle head 28 comprises, atthe proximal end 10 of the torch 8, a refractory material ring 34.

Hence, both abovedescribed torches share specific features, among whicha ceramics coating 44, e.g. of Zirconium oxide (ZrO₂) needs mentioning.This coating may be deposed by a Plasma spray technique, obtaining athickness ranging from 30 to 70 μm, preferably of 50 μm.

For both torches, the electrode head 18 with the toroidal flaps 25 ismade of a highly thermally and electrically conductive material, in thisexample Copper.

The toroidal flap 25 is a means for disposing of the heat from theelectrode to the first cooling circuit, and it is located inside of thelatter.

In particular, the presence of this flap does not merely enable anoverall temperature decrease and a higher heat disposal efficiency, butalso an increase in the exchange surface and a more pronouncedtortuosity of the course enabled to get rid of the degenerativephenomena typical of the anodic torch.

A variant provides that also the electrode head be coated with ahigh-reflectivity metal coating, to further decrease the amount of heatremoved by the cooling water.

Preferably, the refractory material ring 34 defining the mouth of thenozzle 14 is made of Silicon carbide (SiC), whereas the insulator 35 ofthe cathodic torch 8 is made of Aluminium oxide (Al₂O₃).

The presence of this ring enables the latter to act as diaphragm,modifying the electrofluidodynamic conditions of the plasma generatingzone, i.e. at the circuit-making zone.

In fact, the ring steers the trajectory of the plasmogen gas centrewise,forming a plasmogen gas cushion. The preselected material stands out foradequate mechanical strength, high melting temperature and reducedthermal and electrical conductivity.

The addition of the ring increases the stability of the plasma under anyoperating condition, improving the distribution thereof and therebymaking the presence of fluidodynamic disturbances irrelevant.

Moreover, said addition improves the reliability, by avoiding randomelectric arc quenchings between the plasma and the nozzle, and reducesthe energy transported by the refrigerating deionized water, actuallyshielding the nozzle head.

Lastly, concerning the materials, the entire tubular body of the torches8, 10, and in particular the nozzle heads 28 are made of steel,preferably of an AISI stainless steel.

A very important feature of the cathodic (FIGS. 7 and 8) and anodicnozzle head is that of comprising a rounded outer edge 45, in particularto decrease the view factor of the surface of the head directlysubjected to the plasma thermal radiance.

A preferred rounding is apt to decrease said view factor of at least the30%, and up to the 40%.

Always concerning the nozzle head, the replacement of the Copper headwith a stainless steel head facilitates the soldering to the pipes, themalso of stainless steel. The head is sized so as to preserve thefluidodynamic conditions of the cooling water inside of the outsidejacket. However, the head thickness decreases to keep the temperature ofthe outside surface at relatively low values (anyhow higher than thoseof the Copper) that are in no way critical with regard to the mechanicalperformance of the materials.

Thus, it is possible to range from a 150° C. operating temperature (atignited plasma) with the Copper head to a 400° C. temperature with thestainless steel head. The hereto-described innovative interventionscarried out on the torches attain the aims of:

-   -   abating the ordinary torch maintenance costs;    -   increasing the torch reliability and duration; and    -   reducing the energy removed by the torch cooling system,        decreasing the amount of heat removed as well as the quantity of        water utilized.

To the abovedescribed plasma torch a person skilled in the art, in orderto satisfy further and contingent needs, could effect several furthermodifications and variants, all however falling within the protectivescope of the present invention, as defined by the appended claims.

1. A plasma torch, comprising concentric members, including: an outsidejacket ending with a nozzle head; a nozzle formed inside said outsidejacket; and an electrode provided with a respective electrode head, andcomprising respective cooling circuits for a coolant formed inside saidoutside jacket and inside said electrode, each cooling circuits beingformed by a respective reversing pipes having, at said nozzle head andsaid electrode head respectively, an end passage dividing the reversingpipe in a descending section and in an ascending section, wherein, atthe electrode head, the corresponding end passage defines a toroidalslot wherein a toroidal flap is inserted so as to impart a counterU-shaped toroidal course to the end passage, said toroidal flap workingas means for disposing heat from the electrode.
 2. The plasma torchaccording to claim 1, wherein said nozzle, at a mouth thereof, comprisesa refractory material ring.
 3. The plasma torch according to claim 2,wherein said refractory material is silicon carbide.
 4. The plasma torchaccording to claim 1, wherein the outside jacket is coated with aceramic coating.
 5. The plasma torch according to claim 4, wherein saidceramic coating is made of zircon oxide.
 6. The plasma torch accordingto claim 4, wherein the ceramic coating has a thickness ranging from 30μm to 70 μm.
 7. The plasma torch according to claim 4, wherein theceramic coating is a plasma spray coating.
 8. The plasma torch accordingto claim 1, wherein said nozzle head is made of steel.
 9. The plasmatorch according to claim 8, wherein said steel is stainless steel. 10.The plasma torch according to claim 8, wherein the nozzle head has arounded outer edge, so as to decrease at least by 30% the view factor ofthe surface of the nozzle head subjected to plasma thermal radiance. 11.The plasma torch according to claim 1, comprising a central port in thehead of the electrode that enables flow of plasmogen gas and/or ofmaterials to be thermally destroyed.
 12. The plasma torch according toclaim 1, comprising a cathodic electrode having a head with an endportion, wherein the end portion of the head of the cathodic electrodecomprises a metallic material coating having a >1600° C. meltingtemperature.
 13. The plasma torch according to claim 12, wherein saidmetallic material coating is made of tungsten.
 14. The plasma torchaccording to claim 12, wherein said metallic material coating is aplasma spray coating.
 15. The plasma torch according to claim 1, whereinsaid head of said electrode is coated with a metal coating having >0.8reflectivity, said electrode being an anodic electrode.
 16. The plasmatorch according to claim 15, wherein said metal coating having >0.8reflectivity is selected from the group consisting of molybdenum andnickel.